1. Field of the Invention
This invention relates to solid golf balls.
2. Prior Art
It is generally believed that solid golf balls are superior in flight distance performance to wound golf balls, and solid golf balls are widely used in the recent years. Solid golf balls have better flight performance than wound golf balls when hit in the low head speed region, but tend to be rather inferior to wound golf balls in initial velocity and hence, flight distance when hit in the high head speed region, especially the region of head speeds in excess of 48 m/s.
The invention is an improvement in the above-described circumstances, and an object of the invention is to provide a solid golf ball capable of exerting better flight performance than wound golf balls not only in the low head speed region, but also in the high head speed region of head speeds in excess of 48 m/s.
To attain the above object, the invention provides a solid golf ball as defined below.
(1) A solid golf ball having a plurality of dimples formed in its surface, characterized in that at least 70% of the entire dimples have a point of inflection at a position of 75 to 90% of their depth, and the overall dimple volume proportion is 0.7 to 0.9%.
(2) The solid golf ball of (1) having dimples of at least three types which are different in diameter and dimples of at least two types which are identical in diameter, but different in depth, wherein the total number of dimples is 360 to 540.
(3) The solid golf ball of (1) or (2) comprising a solid core formed of a rubber composition to a diameter of 34 to 40 mm and a cover enclosing the core.
(4) The solid golf ball of (3) wherein the cover has an inside cover of 1.0 to 2.0 mm thick and an outside cover of 1.0 to 2.0 mm thick formed of a thermoplastic polyurethane elastomer.
According to the invention, at least 70% of the entire dimples are, as viewed in elevational cross section taken at the center of the dimple, of dual dimple shape having a point of inflection at a position corresponding to 75 to 90% of their depth. In addition to the location of a point of inflection at such a deep position, the overall dimple volume proportion is set at 0.7 to 0.9%. The golf ball is then given better flight performance than prior art wound golf balls, even in a high head speed region in excess of 48 m/s. Also, by selecting the combination of dimple types to be a combination of dimples of at least three types which are different in diameter with dimples of at least two types which are identical in diameter, but different in depth, and the total number of dimples to be 360 to 540; by designing the solid golf ball structure as comprising a solid core formed of a rubber composition to a diameter of 34 to 40 mm and a cover enclosing the core; and by designing the solid golf ball structure such that the cover has an inside cover of 1.0 to 2.0 mm thick and an outside cover of 1.0 to 2.0 mm thick formed of a thermoplastic polyurethane elastomer, better flight performance in the high head speed region is ensured. Especially when some dimples are made deeper, the difference in flight distance between pole hitting and seam hitting is minimized so that the ball has improved symmetry.